The present invention relates to a device for the measurement of angles via a photoelectrically scanned scale, and therefore to a photoelectric position encoder.
Such devices are used, for instance, in machine tools, theodolites, etc., in order automatically to measure the position of a moving machine part and to feed the result of the measurement (which represents the actual position of the machine part relative to a fixed reference) via electronic means to an indicating unit, or to further process the measurement for control of the machine in question.
Known digital measuring devices with photoelectric-scale scanning can be divided into two groups, on the basis of their function and construction, namely, into (1) absolute-measurement systems which use a so-called coded scale and (2) incremental-measurement systems which use a scale having uniform equidistant graduation.
The manufacture of absolute-measuring systems is very expensive, particularly if high resolution over a relatively large measurement range is to be obtained. Regardless of the selected code, such systems require a number a of resolvable steps which is the quotient of the measurement range divided by the smallest resolvable unit, namely, at least .sub.2 log a code tracks on the scale, as well as an equal number of photoelectric devices for reading the scale.
Incremental-measurement systems, on the other hand, are less expensive since scales with only single-track raster division are required; such systems permit, as when used with the moire effect and electronic interpolation, very much greater resolution than absolute measurement systems. However, since the determination of instantaneous position in incremental systems is based on a continuous summation or subtraction of increments traversed, measurement errors may continuously accumulate, as for example by producing an erroneous pulse upon rapid change in the direction of measurement. This has an especially detrimental effect when the measurement system is in quasi-standstill operation.
To avoid this disadvantage, it is known for instance, from West German Auslegeschrift AS No. 2,540,412 in the case of linear measurement systems to arrange, alongside the raster graduation on the scale used, a number of marks which represent absolute values, and in cooperation with a suitable reading unit serve, upon traversing the same, for repeated initiation of the incremental-measurement system. In this way, the accumulation cycle is reduced; but a device equipped with such a measurement system must be brought at regular intervals into a given position for initiation so that the system is burdened with a cumbersome, and in many cases, disturbing manner of operation.
This disadvantage is also present in the device proposed in West German Pat. No. 2,426,212, which has a plurality of absolute marks arranged at a distance apart which is fixed with the precision of the incremental graduation and therefore can be initiated several times over its measurement range. In the region between absolute marks, however, the measurement system behaves like a purely incremental-operating system, since the detector used for the detection of the absolute marks extends merely over the range of a single absolute mark.
The above remarks on the prior art which are directed to linear position encoders apply by analogy also to angular position encoders (angle encoders). In addition, it should be pointed out that angle encoders react sensitively to eccentricities of the circular scale, and that measurement errors resulting therefrom must be avoided by the greater expense of double scanning, using two detectors which are 180.degree. apart, about the axis of the scale.
EP (European Patent Office) Unexamined Application for Patent (Offenlegungsschrift No.) 00 13 799, describes, inter alia a device for measuring angle by means of a scale which is movable relative to a photodiode array in the form of a circular segment, the incremental graduation of which scale is projected onto the photosensitive surface of the array. Scale and array have raster constants which differ from each other in order to be able to carry out a special, expensive digital-interpolation process. Said EP application, further describes an absolute linear-measurement system which consists of a linear photodiode array and a substantially incrementally graduated scale movable relative thereto; the latter scale has a raster subdivided by a plurality of code fields having different digital characterization. The cost of the manufacture of such special scales is very great.